Epoxidation of an Alkene

Epoxidation of an alkene followed by lithium aluminum hydride reduction of the result mg epoxide gives the same alcohol that would be obtained by acid catalyzed hydration (Section 610) of the alkene... 

The Prilezhaev reaction is a rarely used name for the epoxidation of an alkene 1 by reaction with a peracid 2 to yield an oxirane 3. The epoxidation of alkenes has been further developed into an enantioselective method, that is named after Sharpless. 

The lower activity of Ti-beta(OH) in the epoxidation of an alkene containing a polar head (oleic acid, Table XI) was attributed by Blasco et al. (13) to the different adsorption properties of the two catalysts. A strong adsorption of oleic acid through the polar head on the relatively more hydrophilic Ti-beta(OH)... 

A wide range of polymer reagents have been studied [Akelah and Sherrington, 1983 Blossey and Ford, 1989 Ford, 1986a Kirschning et al., 2001], The epoxidation of an alkene by a polymer peracid illustrates the use of a polymer reagent [Frechet and Haque, 1975]. Chloro-methylated polystyrene is treated with potassium bicarbonate in dimethylsulfoxide to yield... 

Fig. 9-1 Scheme for utilization of a polymer reagent. The epoxidation of an alkene by a polymer... 

As already mentioned, the dioxirane epoxidation of an alkene is a stereoselective process, which proceeds with complete retention of the original substrate configuration. The dioxirane epoxidation of chiral alkenes leads to diastereomeric epoxides, for which the diastereoselectivity depends on the alkene and on the dioxirane structure. A comparative study on the diastereoselectivity for the electrophihc epoxidants DMD versus mCPBA has revealed that DMD exhibits consistently a higher diastereoselectivity than mCPBA however, the difference is usually small. An exception is 3-hydroxycyclohexene, which displays a high cis selectivity for mCPBA, but is unselective for DMD . ... 

The most important method of preparation involves oxidation, or epoxidation, of an alkene with a peroxycarboxylic acid, RC03H. This reaction achieves suprafacial addition of oxygen across the double bond, and is a type of electrophilic addition to alkenes (see Exercise 15-53) ... 

The expected enone is obtained in 40% yield. A 15% yield of the product, resulting from hydroxy-directed epoxidation followed by oxidation to ketone, is obtained. A third product, obtained in 30% yield, can be explained by the equilibration of the initially formed allylic chromate ester with an isomeric chromate ester that directs the epoxidation of an alkene, giving an epoxy alcohol that is further oxidized to an... 

Reaction (b) is an epoxidation of an alkene, for which almost any peroxy acid could be used. Peroxybenzoic acid was actually used. 

The epoxidation of an alkene with peracid to give an oxirane. The commercial available mCPBA is a widely used reagent for this conversion, while magnesium mono-perphthalate and peracetic acid are also employed. 

Epoxidation of an Alkene with a Peracid Section 11.9 Ester Condensation Section 20.6 Figure 20.4... 

The epoxidation of an alkene is clearly an oxidation, since an oxygen atom is added. Peroxyacids are highly selective oxidizing agents. Some simple peroxyacids (sometimes called peracids) and their corresponding carboxylic acids are shown next. 

We started this section with a diastereoselective epoxidation of an alkene. The alkene was this one, and it has a substituent cis to the stereo genic centre. We can therefore expect it to have one important conformation, with H eclipsing the double bond. When a reagent—m-CPBA here—attacks this conformation, it will approach the less hindered face, and the outcome is shown. 

Figure 3.5 Epoxidation of an alkene via the in situ generation of a peracid.

Epoxidation of an alkene containing one or more chiral centers can furnish two diastereoisomeric epoxides, depending on the face from which the reagent approaches the ir-bond (equation 2). If the two faces of the ir-bond are unequally shielded, and if polar and stereoelectronic factors are also involved. 

Gal developed a procedure for the determination of the enantiomeric composition of several types of epoxides (222). Nonchiral simple aklyl-amines, for example, isopropylamine, were used in the ring-opening reaction to produce enantiomeric amino alcohols, which were then deri-vatized with the optically active isothiocyanate (21), and the derivatives were resolved by RP LC (222). Despite the two-step derivatization, the procedure is simple and practical to carry out and could be adapted to the determination of the enantioselectivity of the rat-liver-miCTOSomal epoxidation of an alkene (223). 

Woods, K. W., Beak, P. The endocyclic restriction test an experimental evaluation of the geometry at oxygen in the transition structure for epoxidation of an alkene by a peroxy acid. J. Am. Chem. Soc. 1991, 113, 6281-6283. 

Furthermore, pyrazol-3-ones have been oxidized by a variety of oxidizing agents such as ozone in oxygen, hydrogen peroxide solution, 3-chloroperbenzoic acid, aqueous sodium periodate, lead(IV) acetate with boron trifluoride etherate, or atmospheric oxidation. The reactions lead mainly to epoxidation of an alkene or imine functionality and cleavage of the pyrazol-3-one ring. 

The oxygen atom of the OH group of the peroxyacid accepts a pair of electrons from the TT bond of the alkene, causing the weak O — O bond to break heterolytically. The electrons from the O—O bond are delocalized onto the carbonyl group. The electrons left behind as the O—H bond breaks add to the carbon of the alkene that becomes electron deficient when the rr bond breaks. Notice that epoxidation of an alkene is a concerted reaction All the bond-forming and bond-breaking processes take place in a single step. 

Epoxidation in the absence of a metal catalyst is possible. Hydrogen peroxide, in the presence of a nitrile, aldehyde or ketone, or a relatively acidic alcohol (e.g. phenol), can effect epoxidation of an alkene. Peroxy-imidic acids RC(=NH)OOH, formed in situ by reaction of nitriles (RC=N) with hydrogen peroxide, react under mildly alkaline or neutral conditions. For example, 2-aUyl-cyclohexanone was readily converted into the corresponding epoxide with the alkaline reagent, whereas with peroxy-acetic acid Baeyer-Villiger ring-expansion intervenes. The perhydrate 39,... 

For practical purposes, most organic chemists mean by oxidation either addition of oxygen to the substrate (such as epoxidation of an alkene), removal of hydrogen (such as the conversion of an alcohol to an aldehyde or ketone), or removal of one electron (such as the conversion of phenoxide anion to the phenoxy radical). Examples of oxidation reactions of alkenes have been described in Chapter 5, including epoxidation, aziridination, dihydroxylation and Wacker oxidation. This chapter therefore concentrates on oxidations of hydrocarbons, alcohols and ketones. 

Acid-Catalyzed Hydration of 2-Methylpropene 241 6.9 Epoxidation of an Alkene 259... 

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